The invention relates to the use of special brighteners for preparing coating slips, coating slips per se and their use for the production of brightened papers.
Optical brighteners are used mainly for brightening paper or textiles or as an additive to detergents. The brightening of uncoated papers or untreated coating papers can be effected by beater use and/or surface application of optical brighteners, which are usually present for this purpose in dissolved form. In the production of coated papers, the addition of optical brighteners to the coating slip is customary, so that, in the finished coated paper, the optical brightener is also present in the pigment layer applied to the paper. Coated papers are particularly suitable for the production of high-quality prints. In addition to good printability properties, their quality is therefore assessed mainly according to optical properties, such as gloss and whiteness. There is a progressive trend toward coated papers having high whitenesses and therefore the desire for optical brighteners which are as effective as possible as coating slip components.
The most customary and most widely used paper brighteners are those of the formula (I)
in which
    M represents Na, K or optionally substituted ammonium.
If the use of the so-called tetrasulfo type and hexasulfo type shown in formula (I) in paper coating is compared, saturation behavior with respect to the CIE whiteness is found above certain added amounts of brightener of the tetrasulfo type. In other words, when larger amounts are used, no further increase in whiteness is found and there may even be adverse effects on the CIE whiteness. This saturation behavior occurs with the use of the hexasulfo type as a rule only when substantially larger amounts are used compared with the tetrasulfo type. Consequently, higher whitenesses can generally be realized with hexasulfoflavonate brighteners than with tetrasulfoflavonate brighteners. The effect of saturation is also referred to as greening. The greening level, e.g., the point above which the use of increasing amounts of brightener result in virtually no further increase in whiteness, can be derived, for example, from the a*-b* diagram, a* and b* being the color coordinates in the CIE Lab system.
Since the greening in the case of hexasulfo types occurs only when relatively large amounts are used, the hexasulfoflavonate brightener shown in formula (I) and also other hexasulfoflavonate brighteners are particularly suitable for the production of coated, highly white paper. The exact application amounts at which the greening occurs in the case of tetra- and hexasulfoflavonate brighteners depend on the composition of the respective coating slip, inter alia on its carrier content.
On recycling coated papers, for example, for reuse of coated waste in the paper mill, the coated paper is beaten again, the brightener not fixed to the fibers initially going into solution from the coat and partly coating paper fibers. The increased solubility of the hexasulfoflavonate brighteners is disadvantageous in this context, since brightener not fixed to fibers acts as an interfering anionic substance in the circulation water of the paper machine and reduces the effect of cationic paper chemicals, such as retention aids or engine sizes, resulting in additional consumption of these paper chemicals.
There is therefore the desire for improved optical brighteners for brightening coating slips, in particular coating slips, with which higher whitenesses can be realized than with the use of customary di- and tetrasulfo types, such as those shown in formula (I), but which lead to a lower load of interfering substances in the circulation water of the paper machine than hexasulfo types on recycling of coated papers.
The brightening of coating slips based on synthetic co-binders is of primary importance. Natural co-binders, in particular starch, are not very suitable for top coats or single coats, owing to their swelling behavior on contact with aqueous liquids. As a result of the swelling, the quality of the printed image is reduced when printing on the coated paper. Starch is therefore preferably used as a co-binder in preliminary coats in the case of multiply coated papers, whereas synthetic co-binders are preferred in the case of singly coated papers or top coats. In the case of single coats or top coats, the whiteness requirements are generally higher than in the case of preliminary coats.
EP-A 192 600 states that certain polyethylene glycol-containing brightener formulations are particularly suitable as coating slip additives. However, only latex binders in combination with natural co-binders are used explicitly for coating slips.
Surprisingly, it has now been found that a certain class of bistriazinylflavonate brighteners having 2 or 4 sulfo groups meet these requirements in an outstanding manner in coating slip systems which contain synthetic co-binders.